How do i make a dimension be a preset build?

The only way I can think to do this is to make it when you enter the dimension, you summon a whole bunch of structures at specific locations but if it's done that way, the structures will reset every time.

As for the space fluid, you could replace every air block in 3x4x3 cube around the player, but that's really annoying to set up and probably won't look that great so I would force the player in the flying state using the make entity fly(in player tab) procedure block instead of a fluid.